Is a ping test better than the HTTP test in WAN Failover?

Yes and no - ping tests are simpler and more straight forward than the HTTP test, but many network operators block ping requests because they can be used for Denial of Service attacks. In both cases, you should select IP addresses that are external to your network but relatively close to you. As the number of network hops increases, the chances of encountering a bad or slow link increases. When that happens, NG Firewall may interpret it as a network problem and report one of your WAN connections as failing.